Question about exception handlers and prologue sequences

[itsLydt]

Hello, same person as the stack layout question from the other day, hah.
I am working on code to perform a context switch, which I intend to do by using the Systick timer to set a PendSV interrupt. Looking at the assembly generated for the Rust PendSV handler vs the assembly generated for the C version I wrote, it looks like prologue / epilogue is being generated for this handler. I believe the Rust equivalent to C's __attribute__((naked)) is the #[unsafe(naked)] attribute, but I get the error "this attribute is not allowed on an exception handler controlled by cortex-m-rt". Is there a workaround / separate mechanism for registering the exception handler that will allow me to control the prologue and epilogue sequences for this handler?

[itsLydt]

From reading check_attr_whitelist from cortex-m-rt-macros-0.7.5/src/lib.rs, it seems like the attribute should be allowed. Is it because I am using #[unsafe(naked)] instead of plain #[naked]? If I do not wrap the attribute, the compiler complains.

[jonathanpallant]

The macro simply makes an extern "C" function with an unmangled name. You can do that manually and skip using the macro.

See https://github.com/jonathanpallant/pets/blob/ef3e273ddff2fe09d08bae1499132f95044ebbfa/src/asm/eabihf.rs#L26

[itsLydt]

Oh okay. So as long as the name is correct it will link correctly into the vector table basically?

[jonathanpallant]

The cortex-m-rt vector table uses a symbol of that name, and the linker's job is to stick things together based on symbol names, yes.